In the early 1960's metal container end technology was advanced by the development of easy opening container closures. These closures function without need for special opener tools and are now well known in the art. For example, U.S. Pat. No. 2,723,778 describes an easy opening closure with a metallic reusable plug and U.S. Pat. No. 2,849,164 describes a container wherein the opening in the top of the container is covered by a thin strip or plate, the plate being held in position by engagement with a lip. The ready acceptance of easy-opening containers has resulted in extended use of this type container for a substantial number of canned products. The most popular type of closure has a removable or opening section defined by a score line and has a tab secured to the panel which can be manipulated to effect opening. These openings can be effectively used for food products where the entire end panel is removed to dispense the contents (an example would be U.S. Pat. No. 3,366,270) or for beverage products where only a small section of the end panel is displaced to allow dispensing by pouring, as in U.S. Pat. 3,967,752.
In normal can end closure design and manufacture, it is common practice to secure a tab to the end panel by means of a rivet, the rivet being formed from the end panel and the tab used to effect some sort of easy opening feature. The easy opening feature may be of the kind where the entire scored end panel is removed to dispense the contents of the can or only a portion of the scored end panel is displaced to allow dispensing by pouring the contents. In any case the majority of easy opening features make use of some sort of tab to effect the displacement of the scored portion of the end. There exist a wide variety of tab, designs for this purpose. The most common and preferred of which have a tongue portion of the tab surrounding the rivet cut out from the tab itself, the cut terminating near the forward or nose portion of the tab so as to create a hinge. The cut allows for easy lifting of the tab to effect opening. The hinge exists along a transverse line near the termination of the cut portion. The cut portion is rigidly secured to the end panel so as to permit swinging of the tab relative to the panel about the hinge line to effect an opening pressure on the panel.
One disadvantage of this type of tab is in the formation of the cut portion. The cut portion is usually sheared completely through with a punch and matching die cut edge. These cut edges require close clearances usually within a maximum of 0.0025" depending on the stock thickness so that the cut can be made without excess burr. The corners on the cut edges must be maintained sharp to provide a good cut. In addition, the die cut edge usually requires a spring loaded ejector to remove the cut portion. All of these features of normal tab tongue formation are disadvantageous. There exist common problems with tolerances and alignment which increase the cost of tooling. The ejectors can malfunction and cause jams which hamper efficiencies and speeds. The cut edges become dull which will result in lost tab strength. Problems in maintaining the tooling arise particularly when cutting aluminum because the edge of the tool requires sharpening as it becomes blunted. Cutting the rivet island or tongue does not provide the optimum cut condition to ensure tab resistance to tearing. Even with sharp cutters it is impossible to completely eliminate burrs. These burrs are indicative of an unclean cut or a cut which leaves a jagged edge at the sheared portion. A characteristic of this jagged edge is increased stress risers which increase the tendency for a tear to propagate across the cut portion when the cut is placed in shear on opening. This tendency to promote tear propagation is detrimental to tab strength and worsens as the cutters dull.
In particular, this invention relates to a method of forming a hinged connection between the tab and end panel where the tab has increased resistance to tearing, thus permitting the use of lighter gauge or weaker alloy, lower cost tab stock. The implications of being able to use lower cost material for end manufacture are evident when one considers the billions of ends manufactured annually. The savings can significantly improve the profitability of containers in a very competitive market. A major part of many cost reduction efforts is aimed at increasing product strength so that less material can be used. This invention accomplishes this goal. In addition, to allowing the advantage of lighter gauge tab stock this invention will also permit the use of softer alloy tab stock without loss of tab strength. Softer alloys are often used in manufacture of environmental or retained tab ends because they allow the tab to withstand repeated bending without detachment from the end panel.